1. Field of the Invention
The present invention relates to an apparatus for judging a DTMF (dual-tone multifrequency) signal used in a facsimile machine, a multifunctional telephone, or the like.
2. Description of the Related Art
In DTMF pulsing (telephone switching systems), a simultaneous combination of one of a lower group of frequencies and one of a higher group of frequencies to represent each digit or character is utilized. A DTMF signal is a signal used to transmit a telephone number, etc. from a push-button telephone or the like to a telephone exchange. Recently, a DTMF signal has also come to be used for transmission and reception of information after establishing a connection between telephones. Therefore, some facsimile machines and multifunctional telephones for connection to a public switched telephone network are equipped with a DTMF receiver.
In DTMF pulsing (telephone switching systems), one of four lower frequency tones is combined with one of four higher frequency tones. In telephone switching systems, a tone is an audible signal transmitted over the telecommunications network. The following four frequencies f.sub.a to f.sub.d are allocated to the tones of the lower frequency group.
f.sub.a =697 Hz PA1 f.sub.b =770 Hz PA1 f.sub.c =852 Hz PA1 f.sub.d =941 Hz PA1 f.sub.e =1209 Hz PA1 f.sub.f =1336 Hz PA1 f.sub.g =1477 Hz PA1 f.sub.h =1633 Hz
The following four frequencies f.sub.e to f.sub.h are allocated to the tones of the higher frequency group.
For example, the DTMF signal representing the dial number "1" is composed of a combination of tone frequencies of 697 Hz and 1209 Hz. An exemplary frequency spectrum of a DTMF signal is shown in FIG. 10.
FIG. 9 shows an exemplary conventional DTMF receiver. The DTMF receiver of FIG. 9 includes eight band-pass filters 110 to 117 which are connected to an input terminal 100. Each band-pass filter corresponds to one of the above-mentioned eight tone frequencies f.sub.a to f.sub.h. In each band-pass filter, the center frequency of the pass band is set so as to be equal to the corresponding tone frequency. Connected to the band-pass filters are multipliers 120 to 127 by which the intensities of the output signals from the band-pass filters 110 to 117 are squared, respectively. The multipliers 120 to 127 are connected to smoothing low-pass filters 130 to 137, respectively. The output signals from the smoothing low-pass filters 130 to 137 are supplied to level detectors 140 to 147, respectively. The output signals from the level detectors 140 to 147 are input to a coder 150. The coder 150 outputs a code indicating that a DTMF signal has been entered, based on the output signals from the level detectors 140 to 147.
A signal other than a DTMF signal (such as a voice signal or a signal of a white noise, etc.) may be input to the DTMF receiver. The input signal may include tone frequencies of a DTMF signal. Accordingly, even when a signal which is not a DTMF signal is input, the DTMF receiver erroneously outputs a code indicating that a DTMF signal has been entered. This erroneous output code from the DTMF receiver prevents a system which utilizes the DTMF receiver from properly operating.